The system tried to translate Allowed origins are: %s to the german version erlaubte Ursprünge sind: %s, which contains a special char ü. After some research I found out, that the file with the translations is found here: /usr/share/locale/de/LC_MESSAGES/unattended-upgrades.mo.

Why does apt / unattended-upgrades tries to translate english to german, as I have changed the locales already to the correct english values? How can I stop this behaviour?